An ion implantation delamination method (also referred to as the Smart Cut method (registered trademark)), involving bonding a bond wafer into which. ions are implanted and delaminating a portion of the bond wafer, is one of methods that has recently begun to receive attention for manufacturing a bonded SOT wafer. The ion implantation delamination method is an art (See Patent Document 1) to form a bonded wafer having a thin film (an SOI layer) over a base wafer in the following manner: An oxide film is formed on at least one of two wafers, and gas ions, such as hydrogen ions and rare gas ions, are implanted from a front surface of: one of the wafers (the bond wafer) to form a micro bubble layer (an sealed layer) in the interior of the wafer. The surface from which the ions are implanted is then brought into close contact with the other wafer (a base wafer) through the oxide film (an insulator film), and a heat treatment (a delamination heat treatment) is then performed to cleave one of the wafers (the bond wafer) along the micro bubble layer so that the bond wafer is delaminated into a thin film. A heat treatment (a bonding heat treatment) is then performed to strengthen a bond between the wafers. This method makes the cleaved surface (the delamination surface) a good mirror surface, readily manufacturing an SOI wafer having a thin film, particularly an SOI layer, with relatively high thickness uniformity.
However, in case of fabricating a bonded SOI wafer based on the ion, implantation delamination method, a damaged layer formed due to the ion implantation is present on a surface of the bonded wafer after the delamination, and surface roughness is considerable as compared with a mirror surface of a regular product level silicon wafer. Therefore, according to the ion implantation delamination method, it is necessary to remove such a damaged layer and to reduce the surface roughness.
Conventionally, mirror polishing with extremely small polishing stock removal (a stock removal of about 100 nm), referred to as touch polishing, is performed to remove the damaged layer, and others, in the final step after the bonding heat treatment.
However, when polishing including a machining element is performed with respect to the SOI layer, since the stock removal, of the polishing is not uniform, there occurs a problem that film thickness uniformity of the SOI layer achieved by implantation of hydrogen ions and delamination and others is deteriorated.
For the purpose of solving such a problem, a flattening process involving heat treatment at a high temperature has been employed to improve the surface roughness instead of the touch polishing.
For example, Patent Document 2 suggests adding a heat treatment (a rapid heating/rapid cooling heat treatment (an RTA treatment)) in a reducing atmosphere containing hydrogen without polishing a surface of an SOI layer after a delamination heat treatment (or a bonding heat treatment). Further, Patent Document 3suggests forming an oxide film on an SOI layer by a heat treatment in an oxidizing atmosphere after the delamination heat treatment (or the bonding heat treatment), then removing the oxide film, and subsequently adding a heat treatment (the rapid heating/rapid cooling heat treatment (the RTA treatment)) in a reducing atmosphere.
Furthermore, in Patent Document 4, a sacrificial oxidation treatment is performed with respect to a delaminated SOI wafer after a flattening heat treatment in an inert gas, a hydrogen gas, or a mixed gas atmosphere containing these gases, whereby flattening of a delamination surface and avoidance of OSFs can be achieved at the same time.
As described above, since the high-temperature heat treatment is carried out in place of the touch. polishing and the flattening treatment for improving the surface roughness is effected, SOI wafers each having a diameter of 300 mm and excellent film thickness uniformity that a film thickness Range (a value obtained by subtracting a minimum value from a maximum value in a radial direction) of each SOI layer is within 3 nm can be obtained in a high-volume production level by the ion implantation delamination method.
Patent Document 5 discloses a method involving performing the PTA process under a reducing atmosphere containing hydrogen (hereinafter, also referred to as the hydrogen RTA process), disclosed in Patent Document 2, and the sacrificial oxidation process before the flattening heat treatment (a heat treatment under a non-oxidizing atmosphere), disclosed in Patent Document 4, to improve the surface roughness and to suppress the generation of recessed defects.